Blinds do help insulate a window - some far more than others - by trapping a layer of still air against the glass. A lot of heat is lost (and, in summer, gained) through windows, so a blind that slows that exchange can take the edge off a cold room and a heating bill. Here is which blinds actually do it, and how to get the most from one.
How a blind insulates
The mechanism is simple: a blind holds a pocket of still air between the fabric and the glass, and still air is a poor conductor, so it slows heat moving in or out. That means two things matter most - how much air the blind traps, and how well it seals the edges so that air does not just circulate away. A blind that fits tight to the window does far more than one with big gaps around it.
The standout: honeycomb (cellular) blinds
If insulation is the goal, a honeycomb (also called cellular) blind is the clear best choice. Its pleats form rows of sealed air pockets - little built-in cushions of trapped air - so it insulates better than any flat single layer of fabric. It is the one blind type specifically engineered for thermal performance, and the obvious pick for a cold room, a north-facing window or anyone trying to keep heating bills down.
How the other types compare
After honeycomb, mass and a lined construction help. A lined or interlined Roman, or a thick fabric, traps and slows more heat than a thin roller. A wooden venetian closed tight does a little. A thin single-layer roller, and any slatted blind (venetian, vertical) with gaps between the slats, do the least, because they neither add much mass nor seal well. A blackout fabric is not automatically a good insulator - it blocks light, but unless it is honeycomb or lined it traps little air.
Fit matters as much as type
However good the blind, the fit decides how much it delivers. A blind that overlaps the window - a face fit mounted above the reveal, or one with side channels - seals the air pocket far better than a recess blind with gaps each side, where warm air simply slides past the edges. For the best result, fit the blind snug and overlapping, and close it at dusk in winter to trap the day's warmth.
The honest limit
A blind is a useful, affordable add-on, not a substitute for the window itself. The glazing is where the big difference lies - double or secondary glazing will always outperform a blind - so treat a blind as the cheap, easy layer that helps, especially in a rental or an older home you cannot re-glaze. For the most a window covering can do, pair an insulating blind with lined curtains: the two together trap more air than either alone, in winter for warmth and in summer, closed on the sunny side, to keep heat out.